In industrial measuring and automation technology, for producing measured value signals representing measured variables in analog or digital form, measuring systems installed on-site or near to the process—so called field devices—are applied. The particular measured variables to be registered can be, for example, mass flow, e.g. mass flow rate, density, viscosity, fill level or limit level, pressure, temperature, etc. of a flowable—thus, for example, liquid, powdered, vaporous or gaseous—medium, which is conveyed or held in a corresponding process container, such as, for example, a pipeline or a tank. Further examples for such field devices, known to those skilled in the art, are described at length and in detail in, among others, DE-A 39 34 007, EP-A 1 058 093, EP-A 1 158 289, EP-A 525 920, EP-A 984 248, U.S. Pat. No. 3,764,880, U.S. Pat. No. 3,878,725, U.S. Pat. No. 4,308,754, U.S. Pat. No. 4,317,116, U.S. Pat. No. 4,468,971, U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,574,328, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,617,607, U.S. Pat. No. 4,656,353, U.S. Pat. No. 4,768,384, U.S. Pat. No. 4,850,213, U.S. Pat. No. 4,926,340, U.S. Pat. No. 5,024,104, U.S. Pat. No. 5,052,230, U.S. Pat. No. 5,068,592, U.S. Pat. No. 5,131,279, U.S. Pat. No. 5,207,101, U.S. Pat. No. 5,231,884, U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,363,341, U.S. Pat. No. 5,416,723, U.S. Pat. No. 5,469,748, U.S. Pat. No. 5,535,243, U.S. Pat. No. 5,604,685, U.S. Pat. No. 5,672,975, U.S. Pat. No. 5,687,100, U.S. Pat. No. 5,742,225, U.S. Pat. No. 5,742,225, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,959,372, U.S. Pat. No. 6,006,609, U.S. Pat. No. 6,014,100, U.S. Pat. No. 6,140,940, U.S. Pat. No. 6,236,322, U.S. Pat. No. 6,269,701, U.S. Pat. No. 6,285,094, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,397,683, U.S. Pat. No. 6,476,522, U.S. Pat. No. 6,480,131, U.S. Pat. No. 6,487,507, U.S. Pat. No. 6,512,358, U.S. Pat. No. 6,535,161, U.S. Pat. No. 6,574,515, U.S. Pat. No. 6,577,989, U.S. Pat. No. 6,662,120, U.S. Pat. No. 6,769,301, U.S. Pat. No. 6,776,053, U.S. Pat. No. 6,799,476, U.S. Pat. No. 7,200,503, U.S. Pat. No. 7,630,844, US-A 2008/0015799, WO-A 00/14 485, WO-A 00/26739, WO-A 00/36 379, WO-A 00/48157, WO-A 00/67087, WO-A 01/02816, WO-A 02/086426, WO-A 02/103327, WO-A 02/45045, WO-A 2004/048905, WO-A 2005/040735, WO-A 2006/130087, WO-A 2010/014102, WO-A 88/02476, WO-A 88/02853, and WO-A 95/16897. The measuring systems shown therein have, in each case, a physical-to-electrical measuring transducer for registering the particular measured variable(s), as well as, electrically connected therewith, a transmitter electronics, which is most often externally supplied with electrical energy and which has a driver circuit controlling the measuring transducer; as well as having a measuring and operating circuit for producing measured values representing the at least one measured variable.
The measuring transducer is, in each case, provided so as to be inserted into a wall of the container in each case conveying the medium, or into the course of a line, for example, a pipeline, in each case conveying the medium and serves to produce at least one electrical measurement signal representing the at least one measured variable. For such purpose, the particular measuring transducer, and consequently an actuator provided therein, is, during operation of the measuring system, operated by a driver signal—which is, for example, bipolar and/or at least at times periodic—generated by the driver circuit provided in the transmitter electronics, and driven in such a manner, that it acts on the medium in suitably for the measuring, in order to cause reactions there which correspond with the measured variable to be registered, and which are correspondingly convertible into the at least one measurement signal. In such case, the driver signal can, for example, be an analog or also a suitably clocked binary signal correspondingly controlled with regard to an electrical current level, a voltage level and/or a signal frequency. As examples of such active—thus correspondingly converting an electrical driver signal by means of a (for example, predominantly inductive) actuator into a measurable effect useful for registering the measured variable—measuring transducers are to especially be mentioned HF transmitting/receiving transducers working according to the echo principle, or flow-measuring transducers, serving for measuring flowing media, having at least one coil which is driven by the driver signal and producing a magnetic field, and also, for example, measuring transducers of vibration type having at least one vibrating measuring tube, and an electro-mechanical oscillation exciter acting thereon, or at least one ultrasonic transmitter driven by the driver signal, etc. For the purpose of producing measured values representing the measured variable(s) to be registered by means of the measuring system, the at least one measurement signal is, later in the course of the procedure, fed to a measuring and operating circuit, which is provided in the transmitter electronics and, for example, also formed by means of a microcontroller and/or by means of a digital signal processor. For accommodating the transmitter electronics, the field devices additionally include an electronics housing, which, as for example is provided in U.S. Pat. No. 6,397,683 or WO-A 00/36379, is arranged at a distance from the measuring transducer and can be connected with this via only a flexible line, or which, as for example also shown in EP-A 903 651 or EP-A 1 008 836, is arranged directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer.
Measuring systems of the described type are additionally, usually via a data transmission system connected to the transmitter electronics, connected with one another and/or with corresponding process control computers, to which they transmit the measured value signals, e.g. via a (4 mA to 20 mA) current loop and/or via a digital data bus, and/or from which they receive operating data and/or control commands in a corresponding manner. Serving in such case as data transmission systems are fieldbus systems—especially serial fieldbus systems—such as, for example PROFIBUS-PA, FOUNDATION FIELDBUS, as well as the corresponding transmission protocols. By means of the process control computers, the transmitted measured value signals can be further processed, and, as corresponding measurement results, be, for example, visualized on monitors, or, for example, also be converted into control signals for actuating devices, such as, for example magnetic valves, electric motors etc., which serve for process control. Measuring systems of the type being discussed are additionally often embodied in such a manner, that they satisfy the requirements for intrinsic explosion safety. In accordance therewith, the field devices are operated with such a low electrical power that, failing to achieve ignition conditions, sparks or arcs cannot be electrically triggered. Intrinsically safe explosion protection is provided, for example, according to the European standards EN 50 014 and EN 50 020 or the ignition protection type “intrinsic safety (Ex-i)” defined therein, is met when electronic apparatuses, and consequently field devices, are embodied in such a manner, that the maximum electrical currents, voltages and powers occurring therein in no case exceed predetermined electrical current, voltage and power limit values. Namely, limit values are, in each case, selected in such a manner, that, in the case of a malfunction—for instance, a short-circuit—the maximum released energy is not sufficient to produce an ignition-capable spark, and, respectively, the maximum converted electrical power does not exceed 1 W (=Watt). The voltage can be held beneath the predetermined limit values for example via Z-diodes, the electrical current for example via resistors, and the power via a corresponding combination of voltage limiting and electrical current-limiting components. Modern measuring systems of the type being discussed are additionally often so-called two-conductor field devices, that is such field devices, in the case of which the transmitter electronics is electrically connected with the external electrical energy supply only via a single pair of electrical lines, thus forming a current conducting loop, and is flowed through by an electrical supply current fed by the energy supply, and in the case of which the transmitter electronics also transmits the instantaneous measured value via the same pair of electrical lines to an evaluation unit provided in the external electrical energy supply and/or electrically coupled with this. The transmitter electronics comprises, in such case, a so-called two-conductor connection circuit having a series current controller flowed through by the electrical supply current for setting and/or modulating—especially clocking—the supply current as a function of the instantaneous measured value, as well as a parallel current controller for setting a stabilized input voltage serving as an internal supply voltage of the transmitter electronics, or for leading away an excess part of the electrical supply current not instantaneously required for producing the measured values.
Examples for such measuring systems embodied as—in given cases, also intrinsically safe—two-conductor field devices can, among other things, be taken from WO-A 05/040735, WO-A 04/048905, WO-A 02/45045, WO-A 02/103327, WO-A 00/48157, WO-A 00/26739, WO-A 94/20940, U.S. Pat. No. 6,799,476, U.S. Pat. No. 6,577,989, U.S. Pat. No. 6,662,120, U.S. Pat. No. 6,574,515, U.S. Pat. No. 6,535,161, U.S. Pat. No. 6,512,358, U.S. Pat. No. 6,480,131, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,285,094, U.S. Pat. No. 6,269,701, U.S. Pat. No. 6,140,940, U.S. Pat. No. 6,014,100, U.S. Pat. No. 5,959,372, U.S. Pat. No. 5,742,225, U.S. Pat. No. 5,672,975, U.S. Pat. No. 5,535,243, U.S. Pat. No. 5,416,723, U.S. Pat. No. 5,207,101, U.S. Pat. No. 5,068,592, U.S. Pat. No. 4,926,340, U.S. Pat. No. 4,656,353, U.S. Pat. No. 4,317,116, U.S. Pat. No. 3,764,880, US-A 2008/0015799, U.S. Pat. No. 7,200,503, U.S. Pat. No. 7,630,844, WO-A 00/67087, WO-A 2010/014102, EP-A 1 147 841, EP-A 1 058 093, EP-A 525 920 or DE-A 39 34 007. In given cases, as is, for example, described in U.S. Pat. No. 3,764,880, US-A 2008/0015799, U.S. Pat. No. 7,630,844 or WO-A 2004/048905, a galvanic isolation is provided inside of the transmitter electronics, for example, between the internal driver circuit and the series current controller, in order to prevent that possible, not always safely preventable, potential differences between the plant in which the field device is applied and the external electrical energy supply be uncontrolledly removed. Traditionally, such two-conductor field devices are predominantly designed in such a manner, that an instantaneous electrical current level—set to a level between 4 mA and 20 mA (=milliampere)—of the supply current instantaneously flowing in the single pair line serving as part of a current loop also simultaneously represents the measured value instantaneously produced by the measuring system. Consequently, a particular problem of such two-conductor field devices also lies in the fact that the electrical power at least nominally convertible by or to be converted by the transmitter electronics—in the following, “available power” for short—can, during the operation, fluctuate over a broad range in a practically unpredictable manner. Taking this into consideration, in such measuring systems embodied as two-conductor field devices—and consequently such with (4 mA to 20 mA)-current loops—suitable measures are to be taken, in order optimally (consequently with as low a loss as possible) matched to the instantaneous measuring and operational situation, to distribute the available power—which at times even amounts to considerably less than 100 mW—to the individual components, or to electronic assemblies of the measuring system, consequently to the driver circuit and the measuring and operating circuit. In the case of measuring systems designed as two-conductor field device with active measuring transducers, the total electrical power instantaneously converted in the measuring system, such as is discussed, among others, in U.S. Pat. No. 6,799,476, U.S. Pat. No. 6,014,100, US-A 2008/0015799, U.S. Pat. No. 7,200,503, U.S. Pat. No. 7,630,844, WO-A 2010/014102 or WO-A 02/103327, can, for example, optimally be utilized in the measuring system, so that the electrical power converted in the measuring transducer is matched—ad hoc or predictively—to the instantaneously available power, for example, via a correspondingly adapted clocking of the driver signal and/or via lessening a maximal electrical current level and/or a maximal voltage level of the driver signal.
For optimal distribution of the available electrical power in the measuring system, in, for example, US-A 2008/0015799, U.S. Pat. No. 7,200,503, U.S. Pat. No. 7,630,844, the application of two voltage controllers is provided, of which a first voltage controller delivers to a controller output a variable direct voltage for operating the driver circuit, and a second voltage controller delivers to a controller output a direct voltage—which is independent from the aforementioned operating voltage of the driver circuit, and consequently essentially constantly controlled to a predeterminable voltage level—for operating the measuring and operating circuit. By means of the direct voltage or a secondary voltage branching therefrom, an end stage is operated, which converts a control signal, which is present at a signal input, and which is especially bipolar and/or at least at times periodic, into the driver signal for the measuring transducer or its actuator, and consequently acts as a power amplifier for the control signal, wherein, not least of all for the previously mentioned measuring transducer of vibration type, the driver signal is usually to be set as precisely as possible as regards its electrical current level.
Especially for the above-described case, in which, during operation of a measuring system of the type being discussed, the available electrical power is, as a result of a small magnitude of the measured variable to be registered, equally small—for instance, less than 100 mW—the required electrical current level occasionally can no longer be set by means of the end stage, and, in this respect, the driver signal can no longer be delivered with the required signal quality, or a converting of the control signal into a driver signal having said electrical current level could, as a result of back couplings in the assemblage of the voltage controller, lead to an overloading or a destabilizing of the entire internal voltage supply, along with an at times increased power or energy requirement solely for the return of the measuring system, and consequently of the driver circuit and of the measuring transducer, back to a stable working point or operating state.